Source : Webster's Revised Unabridged Dictionary (1913)

Sotto voce \Sot`to vo"ce\ [It.]
   1. (Mus.) With a restrained voice or moderate force; in an
      undertone.

   2. Spoken low or in an undertone.

Source : WordNet®

sotto voce
     adv : in an undertone; "he uttered a curse sotto voce" [syn: {in a
           low voice}]